The Laws of Conservation of Momentum-Energy, and Thermodynamics.
In modern physics since Einstein, we know that energy and momentum are related in just the same way as space and time to make space-time. They unite to form a quantity which does not change called an invariant, the energy momentum. It is from this invariant that one can derive E=MC^2.
The total energy momentum of a system must not change. Energy, momentum and hence mass cannot just appear out of nowhere without explanation. What goes in one end of the tube, must come out the other end of the tube minus an amount of energy used to do work or lost as heat.
In speaking of which the laws of thermodynamics, for the sake of enjoying science fiction reduce boil down to this one fact. Anything that uses a large amount of energy or moves a large amount of mass will generate waste heat, no engine is 100% efficient.
As a non-physicist watching science fiction if a technology requires that material appear out of thin air without a vast amount of energy being put into the system then it is fantasy. Let us consider the cannon explanation of how Star Trek Replicators work.
This is in fact how replicators always worked. They called them protein resequencers or “food processors” before because they were more limited in how they worked. They did not deconstruct the base material down to the atomic level but to the level of certain macro molecules. Namely proteins and sugars were reshuffled to turn waste into food and in doing so some amount of energy is used. Right now, on the International Space Station the water the astronauts and cosmonauts drink are recycled urine. So, if you visit there and have some iced tea, it’s made of their piss.
Anything you see that involves matter energy conversion, but which does not contend with the incredible amounts of energy it takes to turn a kg of massive particles into an equivalent energy of massless particles is fantasy.
The Light Speed Limitation
Nothing can travel through space at a speed which is greater than that of light. This one is simple to understand. It takes light four years to get to the star closest to us. It also takes light eight minutes to get from the Sun to the Earth. Science fiction which does not depict this travel time for light when it comes to communications much less human travel to the stars is fantasy. That is unless they posit a way around this limitation using the rules of Einstein’s General Relativity. In General relativity the speed of space itself is not limited.
Everyone thinks of warp drive. The real marvel and likely to be more possible and practical technology are that of Star Trek’s subspace radio. Subspace radio takes a radio signal and encapsulates it in a pocket of sub space which can travel at any arbitrarily high speed to reach its destination. Often but not always eliminating the light speed delay. In Star Trek this takes a series of relays. Without the relays there is a delay that makes two-way communication difficult. Science fiction which does not explain how communication at interstellar distances is achieved is fantasy.
One area where Star Trek will sometimes violate the light speed limit is in dealing with telepathy. Somehow, they don’t bother about that limitation for their mind reading species. Telepathy itself violates known laws of physics as the intensity of brain waves would drop to near zero outside the head of a living creature. Telepathy is, as far as we know, fantasy.
The Theory of Evolution, Why All the Aliens Would Not Look Human, Or Would They?
Obviously for production convenience and story telling it is easier to write a weekly TV show if all the aliens are basically human beings with different skin, noses, foreheads etc. That said, in real life, intelligent life need not look human at all. One of the most intelligent groups of animals on earth are the Octopuses. They have dexterous tentacles that with enough brain power might be able to use tools. They have a brain which is fundamentally different from ours. Then there is the humble slimemold. Slime molds are a colony of single celled organisms which working together can solve complex problems, run mazes, and even have a memory of sorts. All without having any neural cells or electrical activity we’d recognize.
In this sense the most scientifically accurate aliens in all of Sci Fi were the “Founders”. A shape shifting species most like real life slime mold. They could of course choose to appear in a humanoid form.
Second to that the humanoid species that look very different from humans are most scientific. Such as those found in the bar scenes of Star Wars.
The least scientifically accurate would-be alien species that look like humans, have mammal like hair, and can interbreed with humans. We can’t even interbreed with other creatures on Earth. The very idea of trying is revolting and even mechanically impossible let alone the genetics of the situation. In short, the logical Vulcans make no sense. I love them as a concept, but they make no sense.
UNLESS we were to find out that all life in the universe shares a common origin. An idea called panspermia and a lot of what is known as convergent evolution were to happen. Meaning very different looking life forms in a similar environment adapt in similar ways and can look the same. See for example the Ichthyosaur and the Dolphin.
So, in summary these are the three laws of physics that science fiction will respect but fantasy will completely reject.
Energy and Momentum
The laws of energy momentum conservation and thermodynamics. Fantasy stories will have energy appear from nowhere while science fiction will build a whole story around the fact that energy is a finite resource.
The light speed limit.
Fantasy will ignore this as inconvenient or too hard to think about. Science fiction will make this a center piece of the story at times. The whole story is about overcoming this limit. Getting the subspace/hyperspace radio back online being critical. Getting Warp drive fixed or else they’ll not get home for decades.
Evolution And the Diversity of Life
Fantasy will thumb its nose at such diversity. Not having a diverse cast of humans let alone a diverse cast of wildly different aliens. While this is understandable as the goal of television and movies is entertainment and people need to be able to relate to the characters. Furthermore, the actors are humans in costumes at the end of the day.
Good sci fi will do its best to diversify the cast of aliens that exist or explain why the cast is human centric in some way. Great sci fi will try to imagine aliens that are wildly different from human beings. Especially now that we have convincing computer graphics to make this doable.
Fantasy just won’t address this. For example, numerous stories where everyone in space is a mutant Viking fighting against or with super powered Wakandan warriors clad in Vibrainum armor. That may be fun to watch but it is not science fiction.
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